This invention relates to devices and methods used to measure the amount of liquid mass in a liquid tank used for example in a propellant tank of launch vehicles and spacecraft. The new device structure reduces the tendency of liquid to remain on a liquid sensor due to surface tension during low gravity conditions after the height of the liquid surface has moved below the sensor thereby reducing the likelihood of a false liquid level measurement.
There are currently two primary means for measuring propellant mass in a low gravity condition, as for example, gravity environments of less than 0.1 g. These low gravity conditions may occur during vehicle flight when in space and under reduced vehicle thrust operation such as for low-gravity propellant transfer and depletion dump of propellant. Normally, during powered flight of a vehicle, the gravity sensed by a propellant tank may be above 0.25 g due to acceleration and other factors. However, during low thrust and high altitude conditions a much reduced or low gravity environment may exist for the vehicle propellant tanks.
During a vehicle main engine operation, the acceleration level on a propellant tank may be sufficient such that the fluid physics of the liquid in the tank may be similar to that of normal gravity. Under these conditions, buoyancy may be the dominant force acting upon the fluid and this determines the liquid position within the tank. However, during low gravity vehicle operating conditions, surface tension may cause liquid to remain in locations such as on liquid level sensors after the liquid surface level has been lowered below the sensor tank level location.
One current method for measuring propellant mass in low gravity uses the pressure-volume-temperature (PVT) method equation of state to estimate the volume of gas in the tank. Pressure and temperature are measured to estimate the volume of gas and, using the known propellant tank volume, the liquid volume may be calculated. This method may have significant inaccuracy in liquid mass estimation due to leaks in the system, feed system damage or improper performance as compared to Earth gravity measurements, liquid tank temperature measurements may not be accurately measured, or large temperature gradients existing in the tank.
Another current method of measuring propellant mass in low gravity relies on the calculating or measuring of flow rate of propellant use and then integrating over time to determine the mass of propellant used and thereby the amount of liquid remaining. For this method, commonly known as the bookkeeping method, inaccuracies may result due to difficulty in measuring flow rate accurately, leaks in the system, or large temperature gradients existing in the tank.
In addition, a compression mass gauge liquid tank measurement system may be under development for future use in vehicle propellant tank liquid level measurement. As understood, this apparatus may use a piston incorporated in a liquid tank wall structure. The piston may be cycled for use in determining a tank""s pressure response to small volume changes. The apparatus may be complex, involving moving parts, and may be heavy as compared to the present invention thus reducing reliability and increasing vehicle weight, neither of which are desirable for space launch and flight vehicles.
As can be seen, there is a need for a simple, reliable device to measure liquid level in propellant tanks during low gravity environment conditions.
A propellant tank liquid level sensor rake according to the present invention comprises a mast installed in a tank that may be conformed to the tank curvature and multiple sensor elements attached to the mast. The sensor element comprises an arm having a wicking vane and a sensor.
In one aspect of the present invention, a liquid level sensor rake for measuring propellant liquid surface level in a propellant tank comprises a mast attached to the interior of the tank wherein the mast may be shaped to generally conform to the shape of the tank wall. The mast may be attached at the tank bottom adjacent to the tank outlet and attached to the slosh baffles of the tank side walls. A plurality of sensor elements are attached to the mast and spaced apart one from the other. The sensor elements may have an arm attached to the mast and a sensor attached at the free end of the arm wherein the arm is approximately horizontal relative to the mast and the sensor is approximately vertical thereto.
In another aspect of the present invention, the sensor element may further comprise a vane formed as a portion of the arm wherein the vane is tapered relative to the liquid flow path from the narrow to wider portions of the arm to a bottom edge of the vane. The arm may be attached to the mast at an angle of between about 70 degrees and 90 degrees and the vane may have a bend radius at a vane attachment portion of the bottom edge.
In yet another aspect of the present invention, the sensor element may comprise an arm attachable to a mast wherein the arm is attached at an end at an angle between about 70 degrees and 90 degrees as measured at a top edge relative to the mast. The arm may have a vane that is tapered relative to the liquid flow path from the narrow to wider portions of the arm to a bottom edge and has a bend radius at a vane attachment portion of the bottom edge to form a gradual blend into the mast. A sensor may be attached at a free end of a curved tube that may then be attached to the arm such that the sensor is approximately vertical when the curved tube is attached to the mast.
In a further aspect of the present invention, there is a method for measuring the propellant liquid surface height of a propellant tank comprising the steps of attaching a mast interior and contoured to the shape of a tank; structuring a sensor element to include an arm for attachment to the mast in a generally horizontal orientation; attaching a sensor to the arm at a free end thereof such that the sensor will be approximately vertical when the sensor element is attached to the mast; attaching a plurality of sensor elements to the mast at locations selected for sensing propellant liquid; and monitoring the sensor elements for the presence or absence of propellant liquid.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.